Dislocations greatly and often detrimentally impact the physical and optoelectronic properties of crystalline diamond solids. For example, toughness and/or wear resistance can be affected by dislocation density and direction. Additionally, dislocations can affect the performance of optical or electronic devices based on crystalline diamond material.
Diamond is a material that is renowned for its exceptional hardness and mechanical properties and this has resulted in its use for several applications (e.g. drilling). Dislocations are known to affect these properties and in particular, in homoepitaxial CVD synthetic diamond material, dislocations normally propagate in a direction approximately parallel with the material's growth direction. The resulting parallel array of dislocations is likely to affect the mechanical properties of the material.
Parallel dislocations of significant density, such as those propagating in a <001> direction of a synthetic diamond crystal grown homoepitaxially on a (001) substrate, result in significant strain and therefore birefringence within the synthetic diamond material, which has been shown to reduce its performance for certain optical applications such as Raman lasers (see, for example, “An intra-cavity Raman laser using synthetic single-crystal diamond”, Walter Lubeigt et al., Optics Express, Vol. 18, No. 16, 2010). As such, it would be desirable to lower the overall strain or at least achieve a better distribution of strain within the material to provide better optical performance. High birefringence is observed when the optical viewing axis is the same direction as the line direction for parallel dislocations, i.e. parallel to the growth direction. In optical applications, for simple engineering considerations (e.g. maximising area) it would be conventional to process the material with major faces that are perpendicular to the growth direction. This will result in dislocations which are perpendicular to the major faces of the material and parallel to the viewing axis resulting in high birefringence.
It is also believed that different dislocation types and directions will affect the performance of CVD synthetic diamond devices differently. It is postulated that the ability to select certain line directions of dislocations and not others will allow the optical and/or electronic properties of a diamond-based device to be influenced and optimised for the particular application desired.
In light of the above, one problem to be solved is to mitigate the adverse effect of certain dislocation types and/or directions in single crystal CVD synthetic diamond material, particularly in relation to optical, mechanical, luminescent, and electrical applications.
The aforementioned problem has been at least partially solved in the past by developing methods which reduce the number of dislocations in order to minimize their detrimental effect. For example, WO2004/027123 and WO2007/066215 disclose methods of forming CVD synthetic diamond material with low dislocation concentrations so as to provide high quality optical, electronic, and/or detector grade diamond material. However, it can be relatively difficult, time consuming, and costly to form CVD synthetic diamond material with a low dislocation density.
Notwithstanding other sources of dislocations, two predominant sources of dislocations include: (i) threading dislocations from the substrate to the CVD layer; and (ii) dislocations created at the interface between the substrate and the CVD layer. With reference to (i), vertically slicing a primary CVD layer to reveal a (001) face and growing a secondary layer upon this face results in threading dislocations from the primary to the secondary layer (where the Burger's vector is preserved). Given that the dislocations in the primary layer are of a <001> direction and edge or mixed 45° type, there are a number of permutations of threading dislocations within the secondary CVD layer (see Table 1). However, all of the threading dislocations are in a <100> direction and either edge or 45° mixed type. Accordingly, while this work demonstrates some degree of dislocation engineering, it is limited both in terms of dislocation line direction and type. With reference to (ii), previous studies (see, for example, M. P. Gaukroger et al., Diamond and Related Materials 17 262-269 (2008)) have shown that substrate preparation has a role in determining the dislocation type in CVD layers grown upon standard (001) substrates. Dislocations propagating from surface defects (e.g. a roughly polished substrate) are generally of a 45° mixed type, the most stable dislocation type in (001) growth.
TABLE 1[001] growth upon a (001)-grown vertically sliced primary CVDlayer, showing the various dislocation types if the threadingdislocations of the secondary layer are in a [010] line direction.PrimaryLineBurgersSecondaryLinelayerdirectionvectorVarietylayerdirectionVariety(001)[001][101]Mixed(100)[010]Mixed45°45°(001)[001][011]Mixed(100)[010]Edge45°(001)[001][110]Edge(100)[010]Mixed45°(001)[001][1−1 0]Edge(100)[010]Mixed45°
In light of the above, it should be appreciated that there is a desire to find routes which minimize the impact of dislocation on specific properties such as electronic and optical properties, which may or may not be consistent with a total reduction in dislocation density. For example in some applications (e.g. those requiring mechanical toughness) a high dislocation density might in fact be preferred, but the direction and/or type of dislocations may be critical to the functional performance of the material. There is hence a need to find a route to engineer the type and/or the direction of the dislocations in homoepitaxially grown single crystal CVD synthetic diamond.
It is an aim of certain embodiments of the present invention to at least partially solve the problems outlined above.